The present invention relates to a process for the synthesis of cinnamic esters and substituted cinnamic esters by condensation of aldehyde(s) with carboxylic anhydride(s) catalysed by iron(III) salts.
The direct route to cinnamic esters is possible via the condensation of aromatic aldehydes with acetic esters (Claisen-Schmidt reaction; (Ber. 23, 976 (1890)) and can be carried out in the presence of metallic sodium. Ullmanns Encyclopedia of Industrial Chemistry 6th Edition, Electronic Release, 1999, Vol. A 24, 231-239 and DE 3422412 describe the condensation of benzaldehyde with acetic esters with alkoxide catalyst in alcohol.
In said cases, the catalyst is used in an at least equimolar amount relative to the benzaldehyde and thus significantly increases the preparation costs of the cinnamic ester. Handling of the catalyst is associated with a high hazard potential.
The Perkin reaction (Perkin, J. Chem. Soc. 31,388 (1877)) likewise offers the possibility of preparing cinnamic acid and cinnamic esters by reacting an aromatic aldehyde, such as benzaldehyde, and acetic anhydride with sodium acetate catalysis. Houben Weyl, Methoden der organischen Chemie [Methods of organic chemistry], 4th Ed., Vol. VIII/3, p. 443, Georg Thieme Verlag Stuttgart, 1952 describes the reaction of benzaldehyde with acetic anhydride and sodium acetate in a molar ratio of 1:1.56:0.65 under reflux/16 h. Work-up from water gives cinnamic acid in a yield of 57%.
By heating for 24 hours or by adding pyridine, it is possible to increase the yield to 70-75%.
JP A 76/15,026; CA 1976, 85:14285h describe the reaction of benzaldehyde with acetic anhydride and sodium acetate in a molar ratio of 1:0.5:0.5 and a reaction temperature of 160xc2x0 C./1 h. The yield of cinnamic acid is 30% under these conditions. If sodium acetate is replaced with potassium phosphate or sodium phosphate, the yield increases to 46%.
J. R. Johnson, Org. Reactions I. 210 (1942) describes the reaction of 0.2 mol of benzaldehyde and 0.3 mol of acetic anhydride using 0.12 mol of potassium acetate. The mixture is refluxed for 5 hours at 170xc2x0 C. The reaction mixture is poured onto 1,200 ml of water. Unreacted benzaldehyde is isolated by steam distillation. Activated carbon is added and the mixture is heated. Following filtration, 12-14 ml of conc. hydrochloric acid are added at elevated temperature, and the system is cooled. The precipitated crystals are filtered off with suction and dried. 16-18 g of cinnamic acid are obtained; the yield is 55-60% based on the amount of benzaldehyde used.
P. Kalnin, Helv. Chim. Acta 11, 977 (1928) describes various salts as catalysts. 1 mol of benzaldehyde and 1.5 mol of acetic anhydride are refluxed at 180xc2x0 C. for 8 hours with 0.65 mol of catalyst. The yields of cinnamic acid are, following work-up, for potassium acetate 72%, for potassium carbonate 59%, for sodium carbonate 40%, for sodium acetate 39%, for sodium phosphate 36%, for potassium phosphate 20% and for potassium sulphide 32%. As well as potassium and sodium salts, amines are also possible as catalysts. For 0.33 mol of triethylamine, the yield is 19%.
A disadvantage of said process variants is the large amount of catalyst used, which makes the product expensive and hinders the reaction and also the work-up.
Therefore, an object of the present invention is to find a low-cost catalyst which, in small amounts, permits the preparation of cinnamic esters or cinnamic acid by condensation of aldehyde(s) with carboxylic anhydride(s) in the presence of a catalyst.
A process for the preparation of cinnamic esters and/or cinnamic acid by condensation of aldehydes with carboxylic anhydrides in the presence of a catalyst has been found, which is characterized in that catalysts, or catalyst mixtures, containing iron(III) salts are used.